Pull Stream

Esiur/Data/GVWIE/GroupInt16Codec.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System;
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+
+namespace Esiur.Data.GVWIE;
+
+public static class GroupInt16Codec
+{
+    // ----------------- Encoder -----------------
+    public static byte[] Encode(IList<short> values)
+    {
+        var dst = new List<byte>(values.Count); // close lower bound
+        int i = 0;
+
+        while (i < values.Count)
+        {
+            ushort zz = ZigZag16(values[i]);
+
+            // Fast path: single byte with 7-bit ZigZag
+            if (zz <= 0x7Fu)
+            {
+                dst.Add((byte)zz); // MSB=0 implicitly
+                i++;
+                continue;
+            }
+
+            // Group path: up to 64 items sharing width (1 or 2 bytes)
+            int start = i;
+            int count = 1;
+            int width = (zz <= 0xFFu) ? 1 : 2;
+
+            while (count < 64 && (i + count) < values.Count)
+            {
+                ushort z2 = ZigZag16(values[i + count]);
+                int w2 = (z2 <= 0xFFu) ? 1 : 2;
+                if (w2 > width) width = w2; // widen as needed
+                count++;
+            }
+
+            // Header: 1 | (count-1)[6 bits] | (width-1)[1 bit]
+            byte header = 0x80;
+            header |= (byte)(((count - 1) & 0x3F) << 1);
+            header |= (byte)((width - 1) & 0x01);
+            dst.Add(header);
+
+            // Payload: count ZigZag magnitudes, LE, 'width' bytes each
+            for (int k = 0; k < count; k++)
+            {
+                ushort z = ZigZag16(values[start + k]);
+                WriteLE(dst, z, width);
+            }
+
+            i += count;
+        }
+
+        return dst.ToArray();
+    }
+
+    // ----------------- Decoder -----------------
+    public static short[] Decode(ReadOnlySpan<byte> src)
+    {
+        var result = new List<short>();
+        int pos = 0;
+
+        while (pos < src.Length)
+        {
+            byte h = src[pos++];
+
+            if ((h & 0x80) == 0)
+            {
+                // Fast path: 7-bit ZigZag
+                ushort zz7 = (ushort)(h & 0x7F);
+                result.Add(UnZigZag16(zz7));
+                continue;
+            }
+
+            int count = ((h >> 1) & 0x3F) + 1; // 1..64
+            int width = (h & 0x01) + 1;        // 1..2
+
+            for (int j = 0; j < count; j++)
+            {
+                uint raw = ReadLE(src, ref pos, width);
+                if (width > 2 && (raw >> 16) != 0)
+                    throw new OverflowException("Decoded ZigZag value exceeds 16-bit range.");
+
+                ushort u = (ushort)raw;
+                short val = UnZigZag16(u);
+                result.Add(val);
+            }
+        }
+
+        return result.ToArray();
+    }
+
+    // ----------------- Helpers -----------------
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static ushort ZigZag16(short v)
+    {
+        // (v << 1) ^ (v >> 15), result as unsigned 16-bit
+        return (ushort)(((uint)(ushort)v << 1) ^ (uint)((int)v >> 15));
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static short UnZigZag16(ushort u)
+    {
+        // (u >> 1) ^ -(u & 1), narrowed to 16-bit signed
+        return (short)((u >> 1) ^ (ushort)-(short)(u & 1));
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static void WriteLE(List<byte> dst, ushort value, int width)
+    {
+        // width is 1 or 2
+        dst.Add((byte)(value & 0xFF));
+        if (width == 2) dst.Add((byte)(value >> 8));
+    }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static uint ReadLE(ReadOnlySpan<byte> src, ref int pos, int width)
+    {
+        if ((uint)(pos + width) > (uint)src.Length)
+            throw new ArgumentException("Buffer underflow while reading group payload.");
+
+        uint v = src[pos++];
+        if (width == 2)
+        {
+            v |= (uint)src[pos++] << 8;
+        }
+        return v;
+    }
+}